Recently, a system generally referred to as a "trip computer" has been gathering attention in the automobile industry. A trip computer gives a driver information concerning vehicle's velocity, and direction and distance to destination. As trip computers have been applied to various practiced uses, in particular attention has been focused on direction sensors for knowing the directions of moving vehicles. Thus, various proposals have been made in connection with the structure of trip computers as such structure relates to direction sensing. Mario H. Acuna et al. proposed a dual-axis flux gate magnetic sensor in IEEE Transactions on Geoscience Electronics Vol. GE-7, No. 4 pp. 252-260, October 1969. This sensor comprises an annular magnetic core, an exciting coil wound around the core and a pair of detection coils wound around the core perpendicularly to each other. An alternating current is supplied to the exciting coil.
How such magnetic direction sensor works will be explained referring to FIG. 4 which schematically shows a magnetic direction sensor head. When an alternating current i is supplied to exciting coil 2 wound around annular magnetic core 1, a magnetic field is excited or induced in the core 2. Detection coil 3 is used for detecting the direction of the induced magnetic field. In detection coil 3, a magnetic field Hi, of a given strength shown by the upward arrow is generated on side A, while a magnetic field Hi having the same given strength, shown by the downward arrow is generated on the opposite side B. Since the magnetic fields on side A and opposite side B are of the same strength, but of opposite direction, the magnetic fields offset each other, and therefore there is no net magnetic flux penetrating the detection coil 3 unless there is an external magnetic field. If there is an external magnetic field Ho, the detection coil 3 is subjected at side A and opposite side B to magnetic fields HA and HB which are respectively expressed by the following equations (assuming the upward direction positive): EQU HA=Hi-Ho EQU HB=-Hi-Ho.
These magnetic fields produce magnetic fluxes .phi.A and .phi.B which penetrate the detection coil 3 at A and B, respectively: EQU .phi.A=F(HA)=F(Hi-Ho) EQU .phi.B=F(HB)=-F(Hi+Ho)
The magnetic fluxes .phi.A and .phi.B and the exciting magnetic field Hi have a relationship as shown in FIG. 5. For the sake of simplicity, .phi.-Hi hysteresis curves are shown by parallelograms. It is noted that a .phi.A-Hi curve and a .phi.B -Hi curve are the same in shape; that they are shifted away from the origin by the value of Ho in the positive and negative directions, respectively, and that the .phi.B-Hi curve is turned upside down with respect to the abscissa Hi.
The total magnetic flux .phi. penetrating the detection coil 3 is a sum of .phi.A and .phi.B. Thus, when Hi varies from a sufficiently large minus level to a sufficiently large plus level and then returns to the minus level, the total magnetic flux .phi. changes in a manner shown by the lines I and II in FIG. 6(a). It should be noted that since Hi varies constantly in terms of time, the abscissa of FIG. 6(a) represents time (t).
The output v of the detection coil is shown by the following equation: EQU v.varies.-d.phi.(t)/dt
Since .phi. is a fucntion of time, the output v also varies as a function of time in a manner as schematically shown in FIG. 6(b).
Japanese Patent Laid-Open No. 54-21889 discloses an apparatus for detecting the direction of a magnetic field from the outputs of detection coils. In this apparatus, the outputs pass through a band-pass filter the central frequency of which is two times as large as the frequency f of an exciting current, then an amplifier, and further a synchronous rectifier to convert it into a DC voltage which is sent to a display. A source of the exciting current is connected to a frequency multiplier to send a signal of frequency 2f to the synchronous rectifier through which only either of positive or negative peaks in the output signal can pass.
As a matter of fact, even if an input voltage supplied to the exciting coil 2 in FIG. 4 is precisely in the form of a reactangular wave varying between plus and minus, a current i flowing in the coil 2 has a wave form as shown in FIG. 7(a) which has pulses which have shoulders 5 and 6 at both leading and trailing edges of the pulses. The reason the pulses of the rectangular wave have shoulders is that the magnetic core 1 has a permeability .mu. which varies along a B-H hysteresis curve, whereby the exciting coil's impedance jwL also changes substantially along the hysteresis curve. Thus, the output voltage v varies in a manner as shown in FIG. 7(b).
The output voltage v passes through a filter to eliminate either of positive or negative peaks thereof. For instance, in FIG. 7(b), the negative peaks 7', 8' are cut to permit only the positive peaks 7, 8 to pass. For this purpose, a gate is opened at constant intervals (t.sub.1 .fwdarw.t.sub.2, t.sub.3 .fwdarw.t.sub.4) synchronously with the leading and trailing edges of the rectangular input voltage.
However, since the output signal shows a pair of positive and negative peaks at each of the leading and trailing edges of the input voltage, a signal for opening the gate should have a frequency which is just twice as high as the frequency f of the current supplied to the exciting coil. This necessitates a circuit which supplies a signal of frequency 2f synchronously with an oscillator for the exciting coil. This in turn makes a driving circuit of the direction sensor rather complicated.
In addition, since positive and negative peaks 7, 7', 8, 8' of the output signal are close to each other, the timing of opening and shutting the gate should be extremely precise. Moreover, since the width of shoulders 5, 6 may vary depending on the material and shape of the magnetic core, the setting of the intervals t.sub.1 .fwdarw.t.sub.2 and t.sub.3 .fwdarw.t.sub.4 during which the gate is open results in there being restrictions in materials and designs of the magnetic core. On the other hand, once the magnetic core's material and design is given, the timing of opening and shutting the gate is restricted accordingly.
Furthermore, since the permeability .mu. of the magnetic core varies with the temperature, the width of the shoulders 5, 6 also changes depending on temperature conditions. Thus, avoidance of any errors due to temperature variations requires further precision in gate-opening timing.